Add a BOLT11 invoice utility to ChannelManager

Add a BOLT11 invoice utility to ChannelManager

Now that the lightning crate depends on the lightning_invoice crate, the
utility functions previously living in the latter can be implemented on
ChannelManager. Additionally, the parameters are now moved to a struct
in order to remove the increasingly combinatorial blow-up of methods.

The new Bolt11InvoiceParameters is used to determine what values to set
in the invoice. Using None for any given parameter results in a
reasonable the default or a behavior determined by the ChannelManager as
detailed in the documentation.